1. Department of Economics, Massachusetts Institute of Technology, 50 Memorial Drive, E52-371, Cambridge, MA 02142, and National Bureau of Economic Research (email: )
2. Research Department, Norges Bank, Postboks 1179 Sentrum, 0107 Oslo, Norway, and Statistics Norway (email: )
3. Department of Economics, University of Chicago, 1126 E. 59th Street, Chicago, IL 60637, Statistics Norway, and NBER (email: )
4. Department of Economics, University of Chicago, 1126 E. 59th Street, Chicago, IL 60637 (email: )